In general, organic EL devices, when employing their simplest structure, are composed of a light emitting layer and a pair of opposing electrodes interposing that layer. Namely, organic EL devices utilize a phenomenon by which, when an electric field is applied between both electrodes, electrons are injected from the cathode and holes are injected from the anode and energy is released in the form of light when the electrons and holes are recombined in the light emitting layer.
Excitons generated during this recombination are formed at a ratio of singlet excitons to triplet excitons of 1:3 in accordance with the statistics of electron spin. Organic EL devices of the fluorescence emission type that utilize emission of light by singlet excitons are said to have a limit of internal quantum efficiency of 25%. On the other hand, organic EL devices of the phosphorescence emission type that utilize emission of light by triplet excitons using an iridium complex are said to theoretically enhance internal quantum efficiency up to 100% in the case of having efficiently carried out intersystem crossing from singlet excitons.
More recently, highly efficient organic EL devices have been developed that utilize delayed fluorescence. For example, PTL 1 discloses an organic EL device that utilizes a thermally activated delayed fluorescence (TADF) mechanism. Although this method enhances internal quantum efficiency, further improvement of service life characteristics is required in the same manner as devices of the phosphorescence emission type.
In order to improve the characteristics of organic EL devices, devices are being studied that contain bis-carbazoles or carborane compounds in an organic layer in the manner of the devices disclosed in PTL 2 to PTL 8.